1. Field of the Invention
The invention relates to an apparatus and a method of determining a rotational frequency of an optical disc for enhancement in a transfer rate.
2. Description of the Related Art
In recent CD-ROM drivers and video disc drivers, there is a need for increasing a rotational frequency of an optical disc up to a rotational frequency in the range of 4,000 rpm to 5,000 rpm. The present standard rotational frequency for CD-ROM driver is about 500 rpm. Hence, if a rotational frequency of an optical disc is gradually increased, an oscillation of a track becomes greater, and accordingly, it is quite difficult or almost impossible to stably draw a track in. In particular, an optical disc having a greater eccentricity tends to oscillate more remarkably.
It is prescribed that an allowable eccentricity of an optical disc is equal to or smaller than 140 xcexcm. Since a track pitch in an optical disc is about 1.6 xcexcm, it would be necessary to prepare a device for moving an optical beam spot, which is able to cover an area at least 87.5 times greater than a track pitch (140/1.6=87.5). In addition, when an optical disc is secured to a drive shaft under a certain fitting tolerance, since the drive shaft also has an eccentricity, a total eccentricity would be increased. Accordingly, a servo system for drawing a track therein would be heavily burdened.
The assignee has already suggested a tracking controller in Japanese Unexamined Patent Publication No. 8-96379. This tracking controller satisfies a transfer rate of 140 Mbps which is required for digitally recording incompressive PAL video signals. FIG. 1 illustrates the suggested tracking controller.
An optical head 202 emits a laser beam to an optical disc 201 through a focusing lens 221, and receives a laser beam reflected from the optical disc 201 through the focusing lens 221. The thus received laser beam is introduced to and detected by an optical sensor 222. The optical sensor 222 has a light-receiving plane which is divided into two portions in a track-wise direction of the optical disc 201. The focusing lens 221 is movably supported with an actuator 223.
A subtracting section 203 receives two output signals from the optical sensor 202, and emits a subtraction signal S201 indicative of a difference between the two output signals. An adding section 204 also receives two output signals from the optical sensor 202, and emits an addition signal S205 indicative of a sum of the two output signals.
AGC 205 level-controls the subtraction signal S201 in dependence on the addition signal S205 to thereby absorb a fluctuation in reflectivity of the optical disc 201 for keeping a loop gain constant. An error amplifier 206 compares an output from AGC 205 with a standard input to thereby generate a tracking error signal. A phase compensator 207 gives a suitable frequency-phase characteristic to the tracking error signal. A loop switch 208 operates in response to a tracking start signal S210 emitted from a D-type flip-flop 213. A drive amplifier 209 drives the actuator 223 in accordance with the tracking error signal to thereby cause the laser beam to follow a track.
A relative speed detector 210 is comprised of a binary circuit 101, an edge detecting circuit 102 and a retriggerable monostable multivibrator 103. The relative speed detector 210 monitors a relative speed of the laser beam between a speed in a track-wise direction and a speed in a radius-wise direction of the optical disc. When the relative speed is below a predetermined speed, the relative speed detector 210 emits a speed detecting pulse S204. In other words, the relative speed detector 210 detects a time when the laser beam reaches an intermediate center between a track and a groove of the optical disc 201, and emits the speed detecting pulse S204 when an interval between the detection is longer than a predetermined value. The speed detecting pulse S204 has a predetermined pulse width.
A track detector 211 is comprised of an AC component extracting circuit 111, a binary circuit 112 and a retriggerable monostable multivibrator 113. The track detector 211 monitors a time when the laser beam reaches an edge of a track, based on the addition signal S205, and emits a track detecting pulse S208 having a predetermined pulse width.
The thus generated speed detecting pulse S204 and track detecting pulse S208 are input to an AND circuit 212, which emits a signal S209 indicative of a logical product of the pulses S204 and S208.
The D-type flip-flop 213 receives the signal S209 at a clock input terminal, and also receives a tracking indicating signal S211 at D input terminal, to thereby emit the above-mentioned track start signal S210. Specifically, the D-type flip-flop 213 emits the track start signal S210 just on receipt of the signal S209 when the tracking indicating signal S211 is in H-level, to thereby turn the loop switch 208 on for starting drawing a track in.
The above-mentioned tracking controller illustrated in FIG. 1 can stably draw a track therein under conditions that a rotational frequency of an optical disc is 4500 rpm, an eccentricity of an optical disc is 90 xcexcm, and a track pitch is 1.2 xcexcm.
As mentioned earlier, the tracking controller illustrated in FIG. 1 can deal only with an optical disc having an eccentricity of 90 xcexcm or smaller. However, some optical discs have an eccentricity of 100 xcexcm or greater. The tracking controller illustrated in FIG. 1 would fail to draw a track therein in such optical discs.
Apart from the apparatus illustrated in FIG. 1, many attempts have been made in order to compensate for an eccentricity of an optical disc, as follows.
Japanese Unexamined Patent Publication No. 62-109273 has suggested an apparatus for compensating for an eccentricity of an optical disc, comprising an eccentricity detector for detecting an eccentricity of an optical disc in view of three signals: an output signal emitted from a pick-up position controller for controlling a pick-up to move in a radius-wise direction of a compact disc; an output signal emitted from a pick-up angle controller for controlling an inclination angle of the pick-up to thereby control a position at which the pick-up reads data out of the compact disc; and a signal indicative of a rotation angle of the compact angle, and a memory for storing an eccentricity of the compact disc detected by the eccentricity detector and reading out an eccentricity about a rotation angle.
Japanese Unexamined Patent Publication No. 62-121938 has suggested a servo system comprising an actuator to which feedback servo is applied in order to keep a position relative to a recording track of a rotary recording medium. The actuator is designed to have a transfer characteristic by which a peak value varies in accordance with an eccentric fundamental wave component of the rotary recording medium.
Japanese Unexamined Patent Publication No. 1-184643 has suggested an apparatus for detecting an eccentricity of an optical disc, comprising an optical head including a lens equipped with an actuator for following a track of an optical disc, a first circuit for emitting a first signal indicative of a gap between the track and the lens, a second circuit for emitting a second signal indicative of a rotational period of the optical disc divided by N wherein N is a positive integer, a third circuit for driving the actuator by a lens drive signal having the same period as that of the second signal, and a fourth circuit for calculating an eccentricity of the track in each half of a period of the lens drive signal by means of the first and second signals.
Japanese Unexamined Patent Publication No. 1-256045 has suggested an apparatus for checking an optical disc, comprising first means for detecting an eccentricity of an optical disc, and second means for switching a tracking servo loop of an actuator. The second means releases the tracking servo loop of the actuator when an eccentricity of an optical disc detected by the first means is below a predetermined value, and closes the tracking servo loop when the detected eccentricity of an optical disc is over the predetermined value.
Japanese Unexamined Patent Publication No. 3-25728 has suggested an apparatus for controlably positioning a track of an optical disc, comprising a low-pass filter for detecting low frequency components of a tracking error signal, a sample holding circuit for sampling the low frequency components of a tracking error signal, with an index pulse, and a driver circuit for driving a linear actuator.
Japanese Unexamined Patent Publication No. 3-269844 has suggested an apparatus for detecting an eccentricity of a recording medium, comprising means for turning a tracking error signal indicative of a positional gap between a track and a beam, into a binary code, and means for counting the binary code signals in synchronization with a revolution of the recording medium.
Japanese Unexamined Patent Publication No. 4-241261 has suggested an apparatus for reproducing data out of an optical disc, comprising first means for detecting an eccentricity of an optical disc while the optical disc is rotating at a low speed, and second means for compensating for the eccentricity of the optical disc. The optical disc is fixed onto a disc clamp after the eccentricity has been compensated for by the second means.
Japanese Unexamined Patent Publication No. 4-372776 has suggested an apparatus for storing data into a magnetic disc. An original data and a copy of the original data are stored into four regions of a disc in such a manner that two regions into which the original data is stored face with each other and remaining two regions into which the copy data is stored face with each other. One of the original data and one of the copy data are stored in the regions having a smaller eccentricity than that of the remaining regions.
It is an object of the present invention to provide an apparatus and a method of compensating for an eccentricity of an optical disc mounted on a disc driver, to thereby enhance an average transfer rate.
In one aspect of the present invention, there is provided an apparatus for compensating for an eccentricity of an optical disc, including (a) a device for measuring an eccentricity of an optical disc mounted on a driver for rotation, and (b) a device for determining a rotational frequency of the optical disc in dependence on the thus measured eccentricity of the optical disc.
There is further provided an apparatus for compensating for an eccentricity of an optical disc, including (a) a driver on which an optical disc is mounted for rotation, (b) an optical head for emitting optical beams to the optical disc and receiving optical beams reflected from the optical disc, (c) a mover to which the optical head is secured and which moves in a radius-wise direction of the optical disc, and (d) a controller for controlling the mover to move in the radius-wise direction by a desired distance, and for determining an eccentricity of the optical disc in view of the optical beams reflected from the optical disc, and controlling a rotational frequency of the optical disc in accordance with the thus determined eccentricity.
The apparatus may further include a track cross generating circuit which receives a track error signal from the optical disc and transmits a track cross pulse to the controller.
It is preferable that the controller controls the mover to position at such a location that the mover is remote from innermost and outermost tracks of the optical disc by at least a certain distance when an eccentricity of the optical disc is measured. Herein, the certain distance may be defined as a quotient, A/B, wherein A indicates an allowable eccentricity of an optical disc, and B indicates a track pitch of an optical disc.
In another aspect of the present invention, there is provided a method of compensating for an eccentricity of an optical disc, including the steps of (a) measuring an eccentricity of an optical disc mounted on a driver for rotation, and (b) determining a rotational frequency of the optical disc in dependence on the thus measured eccentricity of the optical disc.
There is further provided a method of compensating for an eccentricity of an optical disc, including the steps of (a) rotating an optical disc, (b) measuring an eccentricity of the optical disc, and (c) rotating the optical disc at a rotational frequency determined in accordance with the thus measured eccentricity of the optical disc.
The method may further include the steps of (d) moving the optical disc in a radius-wise direction thereof onto an intermediate track between innermost and outermost tracks of the optical disc when an eccentricity of the optical disc is measured. The step (d) is to be carried out between the steps (a) and (b).
It is preferable that the optical disc is moved remote from innermost and outermost tracks of the optical disc by at least a certain distance in the step (d). For instance, certain distance may be defined as mentioned above.
There is still further provided a method of compensating for an eccentricity of an optical disc, including the steps of (a) loading an optical disc onto a spindle motor, (b) driving the spindle motor to thereby rotate the optical disc, (c) applying focus servo to a laser beam to be emitted to the optical disc without applying tracking servo to the laser beam, (d) moving the optical disc in a radius-wise direction thereof onto an intermediate track between innermost and outermost tracks of the optical disc, (e) producing a track cross pulse signal in accordance with a track error signal transmitted from the optical disc, (f) calculating an eccentricity of the optical disc based on the track cross pulse signal, (g) determining a rotational frequency of the optical disc in accordance with the thus calculated eccentricity, (h) driving the spindle motor so that the optical disc is rotated at the thus determined rotational frequency, and (i) applying tracking servo to the laser beam for reading data out of and writing data into the optical disc.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.